In order to evaluate the nature of underground formations surrounding a borehole, it is often desirable to obtain and analyze samples of formation fluids from various specific locations in the borehole. Over the years, various tools and procedures have been developed to facilitate this formation fluid evaluation process. Examples of such tools can be found in U.S. Pat. No. 6,476,384 (“the '384 patent”), the entirety of which is hereby incorporated by reference.
As described in the '384 patent, Schlumberger's repeat formation tester (RFT) and modular formation dynamics tester (MDT) tools are specific examples of sampling tools. In particular, the MDT tool includes a fluid analysis module for analyzing fluids sampled by the tool. FIG. 16 illustrates a schematic diagram of such a downhole tool 10 for testing earth formations and analyzing the composition of fluids from the formation. Downhole tool 10 is suspended in a borehole 12 from a logging cable 15 that is connected in a conventional fashion to a surface system 18. Surface system 18 incorporates appropriate electronics and processing systems for control of downhole tool 10 and analysis of signals received from downhole tool 10.
Downhole tool 10 includes an elongated body 19, which encloses a downhole portion of a tool control system 16. Elongated body 19 also carries a selectively-extendible fluid admitting/withdrawal assembly 20 (shown and described, for example, in U.S. Pat. Nos. 3,780,575, 3,859,851, and 4,860,581, each of which is incorporated herein by reference) and a selectively-extendible anchoring member 21. Fluid admitting/withdrawal assembly 20 and anchoring member 21 are respectively arranged on opposite sides of elongated body 19. Fluid admitting/withdrawal assembly 20 is equipped for selectively sealing off or isolating portions of the wall of borehole 12, such that pressure or fluid communication with the adjacent earth formation is established. A fluid analysis module 25 is also included within elongated body 19, through which the obtained fluid flows. The obtained fluid may then be expelled through a port (not shown) back into borehole 12, or sent to one or more sample chambers 22, 23 for recovery at the surface. Control of fluid admitting/withdrawal assembly 20, fluid analysis module 25, and the flow path to sample chambers 22, 23 is maintained by electrical control systems 16, 18.
Over the years, various fluid analysis modules have been developed for use in connection with sampling tools, such as the MDT tool, in order to identify and characterize the samples of formation fluids drawn by the sampling tool. For example, U.S. Pat. No. 4,994,671 (incorporated herein by reference) describes an exemplary fluid analysis module that includes a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber by a fluid admitting assembly are analyzed by directing light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra) in order to characterize the formation fluids. U.S. Pat. Nos. 5,167,149 and 5,201,220 (both of which are incorporated by reference herein) also describe reflecting light from a window/fluid flow interface at certain specific angles to determine the presence of gas in the fluid flow. In addition, as described in U.S. Pat. No. 5,331,156, by taking optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified. As the techniques for measuring and characterizing formation fluids have become more advanced, the demand for more precise formation fluid analysis tools has increased.
As known in the art, the light sources, photodetectors and processing electronics employed in conventional fluid analysis modules are typically adversely affected by the extreme temperatures experienced in downhole environments. For example, the optical power of light sources (such as broad-spectrum incandescent light sources) tends to diminish or drift when operated at elevated temperatures. Similarly, the optical gains of photodetectors, such as Indium Gallium Arsenide (InGaAs) photodiodes, may drift by as much as a few nanometers per Kelvin when subjected to high operating temperatures. Processing electronics, and in particular analog processing electronics, are also known to be susceptible to DC offset drift when operated at extreme temperatures. Because an accurate estimation of the optical density of a formation fluid requires extremely precise measurements, such drifts in the light source, photodetector and/or processing electronics may result in errors in the estimation of the optical density of a formation fluid.
Although various calibration techniques for compensating for these temperature-dependent drifts and shifts are known in the art, these conventional calibration techniques are typically only performed prior to sampling and analyzing the formation fluid. However, because the temperature of the borehole or the sampling tool frequently changes after the calibration operation has been performed, the optical density estimations calculated after such temperature changes may be erroneous. For example, heat from the formation fluid itself, or heat generated by one or more of the components in the sampling tool during its operation, may cause the temperature of the fluid analysis module or sampling tool to change. In addition, because the temperature of the sampling tool itself slowly adjusts to its surrounding temperature due to its relatively large thermal mass, the temperature of the sampling tool and the fluid analysis module housed therein may continue to change even after a calibration operation has been performed. Any such temperature change will likely lead to the aforementioned drifts in optical power, optical gain and DC offset voltage.
Accordingly, there exists a need for an apparatus and method for compensating for temperature drift in the various devices and components used to analyze a downhole formation fluid. More particularly, there exists a need for an apparatus and method capable of continuous calibration.